Purple acid phosphatases (PAPs) catalyze the hydrolysis of a wide range of activated phosphoric acid mono- and di-esters and anhydrides (Klabunde et al., 1996). The PAP proteins are characterized by seven conserved amino acid residues (shown in bold face) in the five conserved motifs XDXX, XDXXY, GNH(D/E), XXXH, XHXH, which are involved in the coordination of the dimetal nuclear center (Fe3+-Me2+) in the active site (Li et al., 2002), where Me is a transition metal; Me2+ is mostly found to be Fe2+ in mammals, and Zn2+, or Mn2+ in plants (Klabunde and Krebs, 1997; Schenk et al., 1999).
Multiple PAP-like sequences are present in plant genomes. In the Arabidopsis genome, twenty-nine potential PAP genes have been identified based on sequence comparison. Most of the functions of characterized plant PAPs are related to phosphorus metabolism. None of the plant PAPs that had been functionally or biochemically characterized carry any transmembrane motif. In addition, no AtPAPs or any other plant PAPs had been discovered to affect sugar signalling and carbon metabolism in plants. Overexpression of AtPAP2 in Arabidopsis, a PAP with a C-terminal motif, can significantly speed up plant growth, increase sugar content in plants and improve seed yield (U.S. Patent Application Publication No. 2010/0159065).